1. Field of the Invention
The present invention relates to a chip type directional coupler employing striplines.
2. Description of the Background Art
In order to manufacture a waveguide circuit, which has been the mainstream of microwave circuits, highly precise machining is required. Therefore, such a waveguide circuit is unsuitable for mass production, high-priced, large-sized and weighty. In a communication device handling frequencies exceeding the UHF level, further, it is difficult to form passive elements such as coils and capacitors into a lumped parameter circuit, since these elements have extremely small values. In a radio set or a BS receiver, therefore, a high frequency circuit is generally formed by a distributed parameter circuit. Microstriplines and striplines, which can be readily implemented in small sizes are employed for forming such a distributed parameter circuit.
A directional coupler is a circuit element which is adapted to draw an output being proportional to only unidirectional power from microwave power flowing through a transmission line without reference to reverse power. FIG. 5 shows a conventional quarter-wavelength coupled-line directional coupler, which is formed by using stripline electrodes 40a and 41a. Referring to FIG. 5, stripline electrodes 40a and 41a partially approach each other in grade separation over a length of .lambda./4, where .lambda. represents a wavelength.
Due to the coupling mode of the portions approaching each other in grade separation over the aforementioned length of .lambda./4. A high frequency signal which is applied to the principal line through a port 1 appears on another port 2, while several-tenth power thereof simultaneously appears on a port 3 of the secondary line. It is possible to arbitrarily set the output level to the port 3 by changing the space between the striplines 40a and 41a. When the space between the striplines 40a and 41a is so set that the output level to the port 3 is half the input level to the port 1, for example, this directional coupler serves as a distributor which equally distributes the input received in the port 1 to the ports 2 and 3 while a signal at the port 3 regularly delays that at the port 2 by a signal phase angle of 90.degree. in this case. Referring to FIG. 5, the stripline electrodes 40a and 41a are shielded by ground electrodes 40b and 41b, shown in two-dot chain lines, which are arranged to hold the stripline electrodes 40a and 41a from upper and lower directions while being insulated from the same.
A well-known directional coupler of such a broadside coupling type is formed by providing striplines on both surfaces of a resin substrate, arranging resin substrates on upper and lower portions thereof and compression-bonding the same through jointing materials, further holding this substance from upper and lower directions by ground plane metal plates, and connecting these layers with each other by screws of the like.
In a digital portable telephone or the like, such a directional coupler is applied to a 90.degree. phase converter or distributor of a phase modulation circuit by utilizing such a 90.degree. phase converting function or distributing function, as a directional coupler 50 shown in FIG. 6. When an output signal of a local oscillator 51 is inputted through a port 1 (corresponding to the port 1 in FIG. 5) of the directional coupler 50, ports 2 and 3 output signals of the same level which are 90.degree. out of phase with each other, and these signals are inputted in mixers 52a and 52b as seen in FIG. 6 respectively. The mixers 52a and 52b as seen in FIG. 6 phase-modulate the local signals by 180.degree. with I and Q pulsing signals respectively. When the modulation outputs are combined with each other in a combiner, the local signals are subjected to four type of phase modulations of 0.degree., 90.degree., 180.degree. and 270.degree..
However, the aforementioned portable telephone has an important subject of miniaturization, and hence further miniaturization is required also for a 90.degree. shifter comprised of the directional coupler. The stripline electrode requires a length of .lambda./4, e.g., 7.5 cm at 1 GHz with a dielectric constant of 1, as hereinabove described. In order to couple linear stripline electrodes having such lengths a substrate having a relatively large volume is required. In the broadside coupling type directional coupler shown in FIG. 5, the coupled lines are positioned along the vertical direction. If the coupled lines are superposed with a plurality of substrates and screwed, therefore, miniaturization of the directional coupler is limited and the cost is increased.